1. Field of the Invention
The present invention relates to a camera in which a camera body thereof is composed of a main body and a mirror box fixed to the main body, and more specifically relates to such a camera in which the mirror box is made of a material having a relatively high thermal expansion coefficient.
2. Description of the Related Art
An SLR camera system in which a camera body is composed of a main body and a mirror box, which are molded independently of each other, is known in the art. In the camera body of this type of SLR camera system, the mirror box which accommodates a reflecting mirror such as a quick-return mirror or a pellicle mirror and is generally equipped with components such as a lens mount, a shutter charge mechanism and a viewfinder optical system, thus required to be formed in a complicated shape, is often die-cast in aluminum or made of a glass fiber reinforced plastic (GFRP). On the other hand, the main body is generally composed of metal plates such as stamped metal plates, so that the mirror box, which is made by joining these metal plates together, is integrated into the main body to constitute the camera body. With the camera body having such a two-piece structure, the mirror box, which is a complex-shaped portion of the camera body, can be easily made while the required strength of the camera body can be easily ensured by the main body.
In such a camera in which the camera body is composed of a combinations of parts made of different materials, the mirror box having a high thermal expansion coefficient (especially the mirror box which is die-cast in aluminum or made of a glass fiber reinforced plastic) is distorted by a variation in ambient temperature by a greater amount than the main body. Such thermal distortion occurs unevenly because the mirror box is fixed to the main body only at finite number of points to thereby cause a variation in amount of thermal distortion of the mirror box at different portions thereof, or because there is a difference in amount of thermal distortion between the portions of the mirror box which are different in direction of resin flow at the process of molding the mirror box in the case where the mirror box is molded of resin. If such a difference in amount of thermal distortion occurs, high-quality images (tightly focused images) cannot be obtained due to such reasons as the mounting surface of the lens mount on the camera body to which a photographing lens is detachably attached becoming nonparallel to a focal plane (a film surface or an imaging surface of an image pick-up device), the axial position of the focal plane (or back focal distance) varying, an image formed on a picture plane partly becoming out-of-focus, and defocusing occurring in the entire area of the picture plane.
Specifically, in a digital camera such as disclosed in Japanese Unexamined Patent Publication No.2003-69886 in which the imaging surface of the image pick-up device such as a CCD image sensor lies in a focal plane, on which an object image is formed through the photographing lens, the mirror box is easily distorted by heat produced by the image pick-up device when the image pick-up device operates. Moreover, in such a digital camera, the positioning accuracy required for the imaging surface of the image pick-up device is so rigorous that requirements for focusing on the image pick-up device cannot be satisfied by merely controlling the amount of thermal distortion by a certain degree, which makes it difficult to obtain high-quality images.
To minimize such thermal distortion, one option would be to make the mirror box out of a material having a low thermal expansion coefficient. However, it is difficult to obtain a material of which a component having a complicated shape such as a mirror box can be easily molded as a light-weight component, and which has a low thermal expansion coefficient.